Kaolinite mobilisation in unconsolidated porous media: Effect of brine salinity and salt type Na- and Ca salts

Abstract

Existence of clay particles in reservoir rock plays a major role in both oil recovery and formation damage. Clay mobilisation and consecutive formation damage have been observed during injection of low-salinity water in oil fields and laboratory coreflood experiments. Hence, this research aimed at understanding and quantifying the effect of clay type, clay content and composition of injected brine on clay mobilisation. In order to study the effect of clay content, several unconsolidated cores using kaolinite and sand are prepared. The clay content of each sample is controlled by mixing an accurately measured mass of kaolinite with sand. A new procedure is developed to assure: a uniform distribution of kaolinite along the core length, reproducible preparation of sand-clay mixture, identical compaction of the mixture in all experiments using axial and overburden stresses, and reproducible permeability data. Each core is initially saturated with high salinity brine (equivalent to sea water salinity) by creating a constant flow rate of 0.6 M solution through the core. The experiments continue with stepwise reduction of salinity of the injected solution (6 steps down to DI water). Around 150 PV of solutions is injected at each step until permeability stabilization. This indicates that no more kaolinite particles are mobilised. Differential pressure across the core is measured continuously and particle concentration and the conductivity of the effluent samples are also measured The kaolinite concentration, solution salinity and valency of ionic species (salt type) are found to be the controlling factors for clay mobilisation. The following correlations are established: relationships between initial kaolinite concentration and initial core permeability, initial kaolinite concentration and degree of permeability damage, and salt type and permeability damage due to salinity reduction. Experimental data show that a core with lower kaolinite content has higher undamaged/initial permeability. It is also observed that the lower is kaolinite content the higher is permeability damage during injection of low salinity water. Significant permeability decline during low-salinity corefloods is due to mobilization of the kaolinite particles and their capture in pore throats. The results also show that injection of solution containing divalent ions (Ca) stabilises the kaolinite particles and prevents their migration during low salinity brine injection.This study is novel in several aspects including: developing a new methodology for unconsolidated core preparation with desired clay content, studying the effect of clay content on initial permeability and severity of formation damage and studying the effect of divalent ions on clay behaviour during low salinity brine injection. The results of this study could be used to engineer the composition of injected water to minimise formation damage based on rock clay content.